Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited.If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

Protocols are what describe the rules
that control horizontal
communication, that is, conversations
between processes that run at corresponding layers within the OSI Reference
Model. At every layer (except layer one) these communications ultimately
take the form of some sort of message that is sent between corresponding
software elements on two or more devices. Since these messages are the
mechanism for communicating information between protocols, they are
most generally called protocol data units (PDUs). Each PDU has
a specific format that implements the features and requirements of the
protocol.

Layer Services and Data Encapsulation

As weve already discussed in
our
look at protocols, the communication between
layers higher than layer one is logical; the only hardware connection
is at the physical layer. Thus, in order for a protocol to communicate,
it must pass down its PDU to the next lower layer for transmission.
Weve also already seen that using
OSI terminology, lower layers are said
to provide services to the layers immediately above them. One
of the services each layer provides is this function: to handle and
manage data received from the layer above.

At any particular layer N, a PDU
is a complete message that implements the protocol at that layer. However,
when this layer N PDU is passed down to layer N-1, it becomes
the data that the layer N-1 protocol is supposed to service.
Thus, the layer N protocol data unit (PDU) is called the layer N-1 service
data unit (SDU). The job of layer N-1 is to transport this SDU,
which it does in turn by placing the layer N SDU into its own PDU format,
preceding the SDU with its own headers and appending footers as necessary.
This process is called data encapsulation, because the entire
contents of the higher-layer message are encapsulated as the data payload
of the message at the lower layer.

What does layer N-1 do with its PDU?
It of course passes it down to the next lower layer, where it is treated
as a layer N-2 SDU. Layer N-2 creates a layer N-2 PDU containing the
layer N-1 SDU and layer N-2s headers and footers. And the so the
process continues, all the way down to the physical layer. In the theoretical
model, what you end up with is a message at layer 1 that consists of
application-layer data that is encapsulated with headers and/or footers
from each of layers 7 through 2 in turn, as shown in Figure 15.

Figure 15: OSI Reference Model Data Encapsulation

Each protocol creates a protocol data unit (PDU) for transmission that includes headers required by that protocol and data to be transmitted. This data becomes the service data unit (SDU) of the next layer below it. This diagram shows a layer 7 PDU consisting of a layer 7 header (L7H) and application data. When this is passed to layer 6, it becomes a layer 6 SDU. The layer 6 protocol prepends to it a layer 6 header (L6H) to create a layer 6 PDU, which is passed to layer 5. The encapsulation process continues all the way down to layer 2, which creates a layer 2 PDUin this case shown with both a header and a footerthat is converted to bits and sent at layer 1.

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!